There are many applications for conduit couplers or connectors, especially where it is desirable to provide a strong mechanical connection without obstructing fluid flow within the conduit. Although the present invention is generally applicable to tubing and hoses of various types, it will be discussed in terms of rigid and flexible metal tubing and hoses associated with exhaust systems of commercial trucking equipment.
The technology of commercial truck exhaust system couplers has experienced an extensive evolution in the recent past. An early technique involved inserting one tubular member into another and securing the two members together by tightening a U-bolt against a semi-circular shoe, using what is known as a quillotine clamp or saddle clamp. This arrangement has the drawback that it deforms the cross-sections of the tubes from circular to elliptical, so that stresses are non-uniform and so that exhaust gas and sound can leak paraxially, i.e., parallel to the axes of the members. A further defect of the arrangement is that the narrow U-bolt frequently dimples the outer tubular member into the inner one, in such a fashion that when the U-bolt is removed it is impossible to disassemble the complete system without cutting the metal. Again, the U-bolt arrangement is very severe in its action on flexible metal hose, while at the same time being relatively inefficient in preventing leaks along the helical grooves in such hoses.
Efforts were made to overcome the defects of U-bolt type clamps by using fairly rigid closed cylinders generally of internal diameter to match the outside diameter of the tubes being clamped, and with means for contracting the coupler cylinder to provide circumferential clamping pressure substantially all the way around its periphery. Such clamps must obviously be positioned at the time the exhaust system is being assembled, although they need not be tightened until later, and, by the same token, they cannot be replaced without disassembling the system.
Other related structures are known, such as that disclosed in U.S. Pat. No. 3,411,748, wherein a relatively rigid split cylinder of appropriate diameter is provided with ears or flanges which are drawn together to produce the necessary circumferential clamping force.
Such heavy gauge or rigid couplers are perceived to have several shortcomings, particularly when used to join tubular members of different outside diameters, as is necessary when one tube is inserted within another. In order to accommodate different outside diameters using heavy gauge clamps, it has been necessary to specially construct stepped clamp diameters to fit the two tubes being joined, one around the outside of the larger tube and one around the outside of the smaller tube where it emerges from the larger tube. Sizes and modifications tend to proliferate in such structures to the point where mass production is no longer economically feasible.
In addition, heavy gauge preformed couplers cannot be efficiently stored or stacked. In factory settings, original equipment manufacturers often prefer to have preformed, substantially cylindrical clamps with which they can quickly and easily couple exhaust tubes. On the other hand, in the field it is preferable to have somewhat flat clamps which have not been substantially preformed for the reason that they can be more readily stored in their flat state, resulting in a higher "packing density" of the clamps.
In response to the problems discussed above, relatively ductile flat clamps or couplers were developed. An example of this type of coupler is disclosed in U.S. Reissue Patent No. 30,042, issued to Hiemstra et al and owned by the assignee herein. Such a flat clamp or connector can be installed in a tubing system after the system has been assembled, and accommodates not only tubes of the same diameter, but also tubes of approximately the same diameter such as telescoped tubes, gripping and sealing both the larger and the smaller tube without any special machining or size selection. Such flat clamps are also easily packaged for shipment and storage since before use they are essentially flat and stack or nest very compactly. Another example of this type of clamp is disclosed in U.S. Pat. No. 4,142,743, issued to McGowen et al. The clamps described in these patents generally include a fairly thin band of ductile metal (e.g., annealed 304 stainless steel) extending between a pair of reinforcement bars. The band is wrapped about the pipe joint and stretched to conform to the pipes as the reinforcement bars are bolted together.
While the ductile metal clamps discussed above are generally very useful for their intended purposes, it is perceived that they possess certain shortcomings for some applications. One problem stems from the fact that the clamps employ 304 stainless steel or the like as the preferred band material. Strictly from a functional standpoint, the ductility of 304 SS makes it an excellent choice: 304 SS can accommodate roughly a 65% increase in length before failing. On the other hand, 304 SS costs about $1.30 per pound. By contrast, 409 SS can only accommodate an increase of 35% in length prior to failure, but costs only about 75 cents per pound.
Another perceived shortcoming of existing ductile metal clamps results from the frictional load on the clamps as they are wrapped about the pipe joint. Friction between the pipes to be joined and the band causes the band to stretch more at its ends than in its central portion. That is, if a clamp having length A is wrapped about a joint having a circumference greater than A, tensile stresses in the band will be markedly concentrated at the end portions of the band near the reinforcement bars. Applying lubricant to the outer surface of the joint in an effort to equalize tension over the entire length of the band is helpful, but inefficient.
One proposed solution to the non-uniform stretching problem discussed above is represented by U.S. Pat. No. 4,466,642, issued to Tonchen. The Tonchen patent discloses a relatively flexible band clamp which is preformed in a central region of the band intermediate the reinforcement bars. The preformed central section approximates a portion of a circle such that it conforms to some degree to the circular configuration of the exhaust pipes to be joined. Since the central section is preformed it is not necessary that it stretch to conform about the pipes. Therefore, the non-uniform stretching problem associated with frictional loads on flat band clamps is, at least conceptually, mitigated.
However, it is perceived that when a central preform clamp like that disclosed in the Tonchen patent is wrapped about a joint a "bulge" frequently develops in the central preformed region. In order to eliminate the bulge, long clamping bolts and relatively large clamping pressures are required.
FIGS. 1A, 1B and 1C show a prior art clamp 20 of the type disclosed in the Tonchen patent. The clamp 20 includes a band 22 provided with reinforcing bars 24a, 24b at the opposite ends thereof. The band 22 is typically of Type 304 annealed soft (non-hardened) stainless steel having a thickness of 0.020 inch and a width of 3 inches. The reinforcing bars 24 are typically made of mild steel. The reinforcing bars 24 have bolt holes 26 suitable for receiving standard hex head bolts 28 and hex nuts 30.
The band 22 of prior art clamp 20 includes a central curved portion 32 disposed between a pair of flat portions 34a, 34b which terminate at the reinforcing bars 25 and are typically welded thereto. As shown in FIG. 1B, reversely curving connecting portions 36a, 36b are transitional between the central curved portion 32 and the flat portions 34. The radius of the central curved portion 32 is typically approximately equal to that of the tubes to be joined by the clamp 20.
FIG. 1C shows a plan view of the prior art clamp 20 in use. The clamp 20 is wrapped about a pair of tubes 40 which are to be joined. The bolts 28 and nuts 30 have been engaged and finger-tightened, leaving a gap between the reinforcing bars 24.
As noted above, when the bolts 28 and nuts 30 are initially assembled, as shown in FIG. 1C, a bulge 42 often develops in the central curved portion 32 of band 22. This bulge 42 can have a bulge height 44 of as much as 1/2 inch.
Considerable effort must be expended to further draw the reinforcing bars 24 together to eliminate the bulge 42 to prevent paraxial sound and exhaust leaks. Friction between the tubes 40 and the band 22 makes it difficult to eliminate this bulge 42 in some cases. It has been determined that this is particularly troublesome when the band 22 of clamp 20 is fabricated using a less ductile metal, e.g., Type 409 stainless steel.
The prior art also includes a second prior art clamp 50, a portion of which is illustrated in FIG. 2. Donaldson Co., Inc., the assignee herein, sells a clamp such as this, referring to it as a "Preformed Clamp". Like clamp 20, clamp 50 includes a thin steel band 52 and a pair of reinforcing bars 54. The bars 54 are apertured to receive a pair of standard carriage bolts (not shown) which are threadedly engaged by hex nuts (not shown). More specifically, reinforcing bar 54b forms a pair of square holes 63 (for the square portions of the carriage bolts) whereas reinforcing bar 54a forms a pair of round holes 60. Carriage bolts are used to simplify the assembly process, necessitating only a single wrench.
The square holes also create significant stress in the area near the holes, however. Stress risers accompany the sharp edges of the square holes, with the result being that the thin steel band 52 can tear near the holes. This problem is depicted in FIG. 2. As can be seen, the band 52 is spot welded to the reinforcing bars 54 at spot welds 62. The spot welds 62 to some degree absorb the tensile stresses introduced in the band 52 during the clamp tightening process, and this stress distribution can be improved still further (at added cost) by wrapping the steel band 52 about the reinforcing bars 54 so that in fact two holes are formed in the stainless steel band 52 for each hole formed by the reinforcing bar 54. See McGowen et al, for example. It has been determined, however, that these techniques do not adequately eliminate high stress areas adjacent the square holes 63. Stress risers occur at the corners of the square holes 63, and tearing 64 of the steel band 52 occurs at the corners of the square holes 63. This problem is thought to be even more severe in connection with less ductile bands 52. Ductile metal bands made of, e.g., 304 stainless steel, can stretch to accommodate or relieve high stresses. However, less ductile metals such as Type 409 stainless steel are less forgiving in terms of tearing.
The present invention addresses the shortcomings of the prior art couplers. In particular, preferred embodiments of the invention address the cost constaints associated with 304 stainless steel clamps; the field storage problem; the "bulge" problem associated with the type of clamp disclosed in U.S. Pat. No. 4,466,642; and the stress concentration problems discussed above.